1. Field of the Invention
This invention relates to a device for detecting the toner concentration of a dry-type, two-component magnetic developer consisting of magnetic carrier powders and toner powders for developing an electrostatic latent image formed on a charge holding body of an electrophotographic copying machine.
2. Description of the Prior Art
Various toner concentration detecting devices for such dry-type two-component magnetic developers have heretofore been known. Known detecting methods may be classified in accordance with operation and principle as follows.
(1) Method for detecting a change of permeability due to a variation of toner concentration of a developer by using a coil.
(2) Method for detecting photoelectrically an amount of the toner adhered to a part of a charged electrophotographic charge holding body or on an electrode surface.
(3) Method for detecting photoelectrically the optical reflectance of the developer or the change of color thereof.
However, in method (1) an extremely small change of permeability must be detected, so that the detecting circuit should be highly sensitive, and it is impossible to detect the toner concentration stably and precisely, because the signal thus obtained is largely varied not only by the change of charged toner amount, but also by the change of fluidity of the developer, and the change of volume and specific gravity caused by stirring. Method (2) requires a structure for adhering toner to the charge holding body or the detecting electrode portion and a photoelectric detecting means, so that the construction becomes complicated, photoelectric detecting surface is further contaminated to reduce measuring precision, or measuring error is caused due to an influence of change of developing efficiency caused by change of toner electrification because of deterioration of the developer. A known toner concentration detecting device relating to method (3) is disclosed in U.S. Pat. No. 3,233,781 in which a transparent window is provided in a wall of the developing apparatus, through which is measured reflectance of a developer, but if toner adheres to the transparent window portion, it is difficult to remove the adhered toner and a large measuring error results. In addition, a device for carrying out said method (3) is also proposed in Japanese Patent Laid-open No. 130,644/77.
However, the toner concentration detecting device proposed in said Japanese Patent Application requires a pressurized air source, so that the device becomes complicated, while the action of the pressurized air source is sometimes insufficient, or if the toner adheres to a photoelectric detector element, particularly to a photoelectric sensor element due to some reason, the photoelectric detecting output is changed and precise measurement cannot be effected. In practice, it is difficult to remove the contamination of the photoelectric detector element. Further, there is a disadvantage in that air flowing out of the housing of the detecting device disperses the toner to contaminate the inside of the copying machine.